mmc: Fix missing 1 ms delay after mmc power up

mmc: Fix missing 1 ms delay after mmc power up

mmc/sd specification requires a 1 ms delay (stable supply voltage)
after vdd was enabled and before issuing first command.

For most sdcard/soc combinations, the missing delay seems to be not a
problem because the processing time between enabling vdd and the first
command is often hundreds of microseconds or more. However, in our
specific case, some sdcards were not detected by u-boot:
* soc: NXP i.MX 93
* sdcards: SanDisk Ultra, 64GB micro SDXC 1,
           MediaRange, 8GB, SDHC
* measured time between vdd and first command: approx. 784us
* symptom: both sdcards did not respond at all to first commands,
           u-boot mmc subsystem ran into timeout and stops to
           initialize the cards

Signed-off-by: Christoph Stoidner <c.stoidner@phytec.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>